Liquid crystal displays are advantageous in being light, thin and low in power consumption, and have been widely used in modern information devices such as notebook computers, mobile phones, and personal digital assistants (PDAs). In order to achieve a wide viewing angle, a number of techniques have been developed, such as Twisted Nematic (TN) LCD with wide viewing film, In-Plane Switching (IPS) LCD, Fringe Field Switching (FFS) LCD, and Multi-domain Vertical Alignment (MVA) LCD.
An MVA LCD is provided with protrusions or slits on its color filter substrate or thin film transistor (TFT) array substrate, which can cause liquid crystal molecules to be aligned in multiple directions to obtain a plurality of different Alignment Domains. The MVA LCD is therefore advantageous in achieving a wide viewing angle.
U.S. Pat. No. 6,922,183 discloses an MVA LCD, a pixel structure of which is shown in FIG. 1. It is noted that the color filter substrate is omitted in FIG. 1 in order to show the structure more clearly. As shown in FIG. 1, in the pixel structure of this LCD, each pixel is divided into two sub-pixels, and the pixel electrode P(m, n) is also divided into two sub-pixel electrodes, that is, a first sub-pixel electrode SP1(m, n) and a second sub-pixel electrode SP2(m, n), which are electrically isolated with a slit 124. The first and second sub-pixel electrodes SP1(m, n) and SP2(m, n) are respectively controlled by two switching elements, for example a first TFT S1(m, n) and a second TFT S2(m, n). The first and second TFTs S1(m, n) and S2(m, n) each have a gate electrode electrically connected with scan lines SL(m) and SL(m+1) respectively, a source electrode electrically connected with data lines DL1(n) and DL2(n) respectively, and a drain electrode electrically connected with the first sub-pixel electrode SP1(m, n) and the second sub-pixel electrode SP2(m, n) respectively.
FIG. 2 is a section view of the pixel structure taken along a line I-I′ of FIG. 1. As shown in FIG. 2, the pixel structure comprises a first substrate 102, a second substrate 122 and a plurality of liquid crystal molecules 126. A black matrix 104 and a color filter layer 106 are formed on a surface of the first substrate 102, and a first insulating layer 108 covers the black matrix 104 and the color filter layer 106. A common electrode 110 is formed on the first insulating layer 108, and is provided with a plurality of protrusions 131. An alignment film 112 covers the common electrode 110 and the protrusions 131.
In addition, the scan lines SL(m) and SL(m+1) are formed on a surface of the second substrate 122 opposite to the common electrode 110, and are covered by a gate insulating layer 120. The data line DL(n) which is not shown in FIG. 2 is formed on the gate insulating layer 120, and is covered by a passivation layer 118. The first sub-pixel electrode SP1(m, n) and the second sub-pixel electrode SP2(m, n) are formed on the passivation layer 118. A plurality of clearances 130 are formed respectively above the scan lines SL(m) and SL(m+1) to separate the pixel electrodes of two adjacent pixels. A slit 124 is provided between the first sub-pixel electrode SP1(m, n) and the second sub-pixel electrode SP2(m, n) to electrically isolate the two sub-pixel electrodes. An alignment film 116 covers the first and second sub-pixel electrodes SP1(m, n) and SP2(m, n). The liquid crystal molecules 126 are sealed between the first substrate 102 and the second substrate 122.
Data signals of opposite polarities are inputted to the first and second sub-pixel electrodes SP1(m, n) and SP2(m, n) through the first and second TFTs S1(m, n) and S2(m, n) respectively to drive the whole pixel. The voltage applied on the common electrode 110 serves as a common voltage Vcom. A voltage higher than the common voltage Vcom is defined as a voltage of positive polarity, and a voltage lower than the common voltage Vcom is defined as a voltage of negative polarity. When the pixel is selected and the first and second TFTs S1(m, n) and S2(m, n) are conducted, a data signal with a voltage of positive polarity +V and a data signal with a voltage of negative polarity −V are respectively inputted to the first sub-pixel electrode SP1(m, n) and the second sub-pixel electrode SP2(m, n). In addition, the difference between the voltage of positive polarity +V and the common voltage Vcom is substantially equal to that between the common voltage Vcom and the voltage of negative polarity −V such that the same gray level is displayed in the two sub-pixels.
When voltages with different polarities are applied on the first and second sub-pixel electrodes SP1(m, n) and SP2(m, n), due to the clearance 130, the slit 124 and the protrusions 131, the liquid crystal molecule 126 forms two display domains with opposite visual properties, and thus an MVA LCD with a wide viewing angle is achieved.
In manufacturing an LCD, dot defects often occur due to failure of a TFT, shorting or the like. The pixel described above consists of two sub-pixels. When a dot defect occurs in one sub-pixel due to failure of a TFT, shorting or the like, the other sub-pixel can still operate normally, and thus the whole pixel does not thoroughly fail. In other words, the dot defect may be “partially repaired” to some extent. In this case, however, normal display cannot be achieved in the whole pixel, so such a “repair” cannot make a complete repair to the dot defect. A similar disadvantage also exists in other LCDs in which a pixel consists of multiple sub-pixels in addition to MVA LCD described above.